Description and Validation of the Simple, Efficient, Dynamic
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Description and Validation of the Simple, Efficient, Dynamic, Global, Ecological Simulator (SEDGES v.1.0) Pablo Paiewonsky1 and Oliver Elison Timm1 1Department of Atmospheric and Environmental Sciences, State University of New York at Albany, 1400 Washington Ave., Albany, NY 12222 Correspondence to: Pablo Paiewonsky ([email protected]) Abstract. In this paper, we present a simple dynamic global vegetation model whose primary intended use is auxiliary to the land-atmosphere coupling scheme of a climate model, particularly one of intermediate complexity. The model simulates and provides important ecological-only variables but also some hydrological and surface energy variables that are typically either simulated by land surface schemes or else used as boundary data input for these schemes. The model formulations and 5 their derivations are presented here, in detail. The model includes some realistic and useful features for its level of complexity, including a photosynthetic dependency on light, full coupling of photosynthesis and transpiration through an interactive canopy resistance, and a soil organic carbon dependence for bare soil albedo. We evaluate the model’s performance by running it as part of a simple land surface scheme that is driven by reanalysis data. The evaluation against observational data includes net primary productivity, leaf area index, surface albedo, and diagnosed variables relevant for the closure of the hydrological cycle. 10 In this set up, we find that the model gives an adequate to good simulation of basic large-scale ecological and hydrological variables. Of the variables analyzed in this paper, gross primary productivity is particularly well simulated. The results also reveal the current limitations of the model. The most significant deficiency is the excessive simulation of evapotranspiration in mid- to high northern latitudes during their winter to spring transition. The model has relative advantage in situations that require some combination of computational efficiency, model transparency and tractability, and the simulation of the large 15 scale vegetation and land surface characteristics under non-present day conditions. 1 Introduction Simulation of the land surface is a critical component of models of the Earth climate system. In such models, heat, energy, and moisture transfer between the land and atmosphere or ocean are calculated with information from the hydrological and vegetation components of these models. SEDGES simulates the gross properties of vegetation, as well as some large scale land 20 surface characteristics, which are important for the simulation of energy and moisture exchanges with the atmosphere. These simulated variables are to be used by a land surface scheme in which SEDGES is embedded in a climate or Earth system model. Models of similar complexity as SEDGES exist that simulate the vegetative if not also the hydrological and energy-transferring aspects of the land surface include VECODE (Brovkin et al., 1997, 2002) and ENTS (Williamson et al., 2006). The purpose of these kinds of models is to efficiently and reasonably simulate dynamic land surface characteristics and behavior and provide 1 these to the atmospheric components of Earth system models of intermediate complexity. In such a framework, the need for sophisticated simulations of the land surface is obviated by the simplifications present in the other model components (which reduce the benefit of more realistic representations of land surface processes as compared to when the other model components are complex and realistic), by a desire to more easily understand the processes underlying experimental results, and by the 5 computational burden that goes along with the increased complexity. SEDGES is a new model that is based on the original SimBA model (Kleidon, 2006b), which was coupled to the Planet Simulator (PlaSim) general circulation model (GCM) (Lunkeit et al., 2007) and even more strongly based on a later version of SimBA that the lead author of this paper developed (Lunkeit et al., 2011), and which was also coupled to PlaSim. Neither version of SimBA has been thoroughly evaluated. However, when coupled to PlaSim, the earlier version’s net primary pro- 10 ductivity was shown to be broadly reasonable in (Kleidon, 2006b), and it has been used successfully in a number of studies (e.g., Kleidon, 2006a, b; Bowring et al., 2014). The SimBA model is based on a light and water-use-efficiency model for crops (Monteith et al., 1989) that was later adapted and expanded to forest canopies (Dewar, 1997). This approach to vegetative productivity is maintained in SEDGES and helps form the core of the model. Other aspects of SEDGES’s structure that are particular to SEDGES (i.e., are uncommon in or absent from other models) are carried over from the original SimBA, including 15 a simple and direct determination of soil water holding capacity and moist-soil leaf cover fraction by the amount of vegetation biomass (Fig. A3). SEDGES builds upon SimBA by improving most of its parameterizations. Some of the updated and expanded model features are taken or adapted from currently existing models. Although SEDGES makes many improvements upon the original SimBA, it is still a simple model; i.e., it is not of "intermediate" complexity (c.f. Willeit and Ganopolski, 2016). Compared to the original 20 SimBA, SEDGES (and the land surface framework that it presupposes) has four major increases in complexity: separation of evapotranspiration into soil and vegetative components, inclusion of aerodynamic conductance in the formulation for carbon uptake by vegetation, full coupling of photosynthesis and transpiration through interactive canopy conductance, and soil organic carbon-dependent soil albedo. Choosing the appropriate level of complexity of represented processes within a land surface or dynamic global vegetation model often depends on the context in which the model is to be used and involves subjective 25 weighting of tradeoffs between increased accuracy and realism on the one hand and, on the other hand, robustness to the chosen values of poorly constrained parameters and model reliability in a wide range of situations (Prentice et al., 2015). This paper presents the SEDGES model’s structure, equations, and ability to simulate ecological and hydrological variables when used in conjunction with a simple soil hydrological scheme and parameterization for aerodynamic conductance and forced by reanalysis data. 30 2 Model Description: SEDGES 2.1 Overview of SEDGES SEDGES is a simple, dynamic global vegetation model (DGVM). Within the context of the history of land surface modeling, the SEDGES framework (defined as SEDGES and the type of land surface model that it presupposes that it forms a part of) 2 combines aspects of 1st and 3rd generation models (Sellers et al., 1997; Pitman, 2003; Prentice et al., 2015), which include, respectively, the use of a simple single-layer "bucket" model for soil hydrology and the full coupling of photosynthesis and transpiration through interactive canopy conductance. The intended main use of SEDGES is for it to be embedded within a land surface scheme and to thus help the land surface 5 scheme simulate large-scale properties and behavior of the surface in which vegetation and soil play a role. These large-scale properties and behavior include interactions between the land surface and the atmosphere through simulated CO2, moisture, en- ergy, heat, and momentum exchanges. SEDGES either directly simulates or else provides the greater land surface scheme with the necessary variables. SEDGES also simulates ecological variables (e.g., biomass, soil organic carbon, forest cover fraction, leaf cover fraction) that are not directly used outside it by either the rest of the land surface scheme or by the atmosphere, but 10 these ecological variables almost always have downstream impact on the simulations of the aforementioned land-atmosphere exchanges. SEDGES has been designed, in particular, for coupling with the Planet Simulator, and thus presupposes that the land surface scheme that it forms a part of be similar to that of PlaSim in its hydrology and scheme for surface evaporation. This section presents, in detail, the SEDGES model formulations for its output ecological and land surface variables, including their derivations. 15 SEDGES is designed to couple with the Planet Simulator GCM, which uses a bulk aerodynamic formulation with a single tile (i.e. one surface type per grid cell). SEDGES handles sub-grid scale heterogeneity according to what is called the "average parameter" method (Giorgi and Avissar, 1997) or the "composite" approach (Li and Arora, 2012; Melton and Arora, 2014), in which land surface characteristics are aggregated to provide a single, representative value at the scale of the entire grid cell. The framework for evapotranspiration (ET) is special, because it also qualifies as a simplified mosaic (or "mixed" (Li and 20 Arora, 2012)) approach, such that only surface conductance differs between the surface types (or tiles). This mosaic approach is also used to extend a "big leaf" formulation for vegetative CO2 uptake (Appendix B of Raupach, 1998) to a mixed soil- vegetation surface, enabling us to isolate the fraction of total ET that is due to transpiration and thus formulate vegetative control over water loss and CO2 uptake. The SEDGES framework neglects evaporation from intercepted canopy water and thus only distinguishes between two tiles when snow is absent: bare soil and vegetation.